Filtered thyratron control circuits



April 1951 c. IZENOUR 2,548,887

' FILTERED THYRATRON CONTROL cmbun's Filed Oct. 27, 1949 V INVENTORGEORGE C. IZENOUR ATTORNEY I Patented Apr. 17, 1951 UNITED STATES PATENTOFFICE FILTERED THYRATRON CONTROL CIRCUITS '5 Claims. 1 The presentinvention relates to alternatingcurrent lighting circuits includingphase-responsive grid-controlled gaseous electron-discharge tubes, orthyratrons, as in my Patent 2,463,463

trol the saturation of an iron-core reactance that is in series with alamp or lamp bank; or in another circuit the thyratron is connected inseries with the lamp or lamp bank. In both types of lamp circuits anadjustable control circuit is provided for the thyratron grid, forfiring the thyratron at a time in each successive cycle that depends onthe desired lamp intensity. Early firing produces bright illumination,whereas tardy firing results in lamp dimming.

A plurality of circuits for difierently controlled lamps are commonlyrequired in theatre lighting. The early firing of thyratrons in onelighting circuit erratically tends to trip other thyratrons set forlater firing. This premature, sporadic discharge results from transientpulses produced by the abrupt current-change in earlyfired thyratrons,from non-conducting to conducting state. The efiect is found in bothtypes of lighting circuit, although it is more prominent in the directseries connection of thyratron and lamp than in the indirect controlcircuit that utilizes a saturable reactor.

' In an effort to meet the transient-flicker problem of such systems,filtering devicesv have been considered to suppress the transients, suchas low-resistance chokes in series with the lamps.

This has various disadvantages, including the high cost, and the greatweight and bulk of such devices which must pass the heavy current of thelamps. Furthermore, a lamp of onlyone power level is suitable to anygiven transient filter, smaller lamps being inadequately filtered andlarger lamps causing excessive voltage drop in the filter.

Rather than to attempt the suppression of the transients, the presentinvention achieves stability of control by preventing transients fromaffecting the control circuits. A low-pass filter is. interposed, in theillustrative embodiment of the invention described in detail below, ineach control circuit to attenute high-frequency transients produced inthe power line by earlier-fired thyratrons of other lighting circuits;and the filter is subdivided to include a choke in series. with the A.C.leads of thephase-control circuit, and

a broad-band low-pass resistance-capacitance filter at the output of thephase-control circuit. The choke is of such inductance as not to shiftthe phase of the control-circuit supply excessively relative to thelamp-circuit supply, but because of the low power requirements of thecontrol circuit, its impedance can nevertheless be substantial. Abroad-band, low-pass filter of the resistance-capacitance type isinterposed between the control circuit and the thyratron grids toincrease the filtering effect, the subdivision of filters achieving thedesired result without appreciably restricting the range of the controlcircuit. Both forms of filter are in the low current-level part of thesystem, rather than in the high current part, and hence are of muchsmaller size and greater eificiency, .and can be of more elaborate,effective design Without excessive cost. Both forms of filter areconnected in that circuit which is energized by the power line but isconnected between the load circuit and the control electrode of thethyratron. And by using an inductance as the filtering impedance in theA.-C. supply leads of the adjustable phase-shifter of theresistance-capacitance type having a resistive voltage divider, thenecessary broad latitude of control is preserved for varying thelighting from full brilliance to extinction.

The invention will be more fully appreciated from the following detaileddescription of an illustrative embodiment in which reference is made toaccompanying drawings.

Fig. 1 is the wiring diagram of an illustrative two-lamp systemembodying features of the invention, and v Fig. 2 is a diagram of analternating-current wave-form of line voltage, including a typicaltransient.

In Fig. 1, a pair of terminals Ill, l2 represent an alternating currentsource that may include transmission lines, transformers, etc., typicalof commercial power mains. To this a series circuit is connectedincluding thyratron l4 and lamp load IS. The. term thyratron is hereused generically to include. gaseous electron-discharge tubes havincontrol electrodes, which electrodes are herein termed grids. A secondthyratron I8; is reversely connected in parallel with thyratron l4 forpotentially full-cycle conductivity, the cathode of each thyratron tothe plate of the other in an arrangement conveniently termedback-to-back connection.

An alternating-current voltage is impressed between the thyratron grids;and cathode... having a controlled, adjustable phase in relation to thephase of the thyratron-lamp circuit, to adjust the light intensity. Anyinst-ability of this circuit may cause flickering or even unintentionalsustained lighting. Transient spikes (Fig. 2) resulting in the powerline from the abrupt firing of the thyratrons in an associated lightingcircuit of the system, tend to produce such effects.

Rather than to insert bulky and costly chokes in series with the lamploads where they must necessarily lose power, and where they are limitedto lighting loads of a certain rating, I have isolated the thyratroninput electrodes from the effects of the transients by a filter of suchcharacter as not to limit the range of the adjustable phase-shifter.Choke 2D is interposed between the input of the adjustable phase-controlcircuit 22 and the connections of the thyratron-lamp circuit to thesupply line. The choke should not be so large as to limit the necessaryrange of the phase-shifter, but it can nevertheless be quite large.Additionally low-pass filter 24 is interposed between the output of thephase-control circuit 22 and the thyratron input electrodes. This filteris of special importance because of the unfiltered input that controlsthe phase-shift circuit, as will be seen. This filter is of the piresistance-capacitance type, including resistor 25 and capacitors 28 and30 for tube Hi. The resistor serves the further purpose of limiting thethyratron grid current in the intervals when the grid of tube I4 ispositive in respect to its cathode. Tube l8 similarly has a pi-typelow-pass filter 2411, including resistor 26a and condensers 28a'and 38a.

The particular phase-shifter shown is that disclosed in greater detailand claimed in my issued patent mentioned above. It includes a"resistive voltage divider 32, 3B in one circuit and a condenser 36 anda grid-controlled back-toback pair of vacuum tubes 38 in a secondcircuit paralleling the voltage divider. An output transformer 39 hasits primary winding connected between intermediate points of these twocircuits and has a separate secondary winding for each thyratron. Anadjustable alternating-current source of grid-control voltage isprovided for the back-to-back vacuum tubes, including potentiometer 40and transformer 42 that has a separate secondary winding for eachvacuum-tube grid. Varied input voltage from potentiometer it to thevacuum tube grids changes the plate resistances, and shifts the phase ofthe voltage to transformer 39. The vacuum tubes have their grids drivennegative during half-cycles when the respective plates are positive, byproper phasing of the secondary windings of transformer 42.

The order of magnitude of the components in relation to each other isgiven in the following example: Choke 30, 7 henries; resistors 32 and34, 5,000 ohms; resistors 26 and 26a, 50,000 ohms; condensers 28 and28a, .004 mid; condenser 30 and 30a, .002 mfd; tube 38, 6SL7 or 5691;condenser 36, 0.25 mfd; resistor M, 50,000. The latter is a trimmer, toadjust the range of the phaseshift network.

A second lighting and control circuit, the dup'licate of that described,is shown in Fig. 1. This circuit has parts with primed numeralscorrespending to the parts in the circuit described, and

is connected to the same line terminals Ill, I2. The two lightingcircuits will normally be adjusted differently (for differentintensities of the two lamps l6 and I6) as may be required to designatedmaster.

light a set on a theatre stage. Transients produced by one lightingcircuit are not suppressed, but are permitted to reach the line. Thetransients are not allowed to affect the ultimate control because of thefilters described that isolate the thyraton grid circuits from thethyratron load circuits. I have discovered that transients of thisnature are damped in comparatively short feeder lines so that they donot noticeably affeet equipment outside the immediate lightinginstallation.

The two phase-shift networks are controlled separately by"individualpotentiometers 40 and 40. These are to be pre-set in uses as describedin my aforementioned patent, and are proportionally dimmed by a commonpotentiometer 46 It is notable that the master is energized directly bythe power line where transients are to be expected to be troublesome;but such transients are attenuated in the phaseshift network, andwhatever the extent that they may be transmitted through the phase-shiftnetwork, they are suppressed by the pi resistancecapacitance filters 24and 24a.

What is claimed is:

1. An alternating-current lighting circuit including a lamp, a thyratronconnected in control relation to said lamp and to power lines, awiderange phase-shift circuit connected to said lines, said phase-shiftnetwork including a tapped resistive voltage divider, an adjustableseries resistive-capacitive phase-shift circuit in parallel with aidvoltage divider, and a transformer having its primary winding connectedbetween said resistive voltage divider and said phaseshift circuit, anda secondary winding connected to the control grid of said thyratron, achoke in the connection between said power line and said phase-shiftnetwork, and a resistor-capacitor low-pass filter in the connectionsbetween said secondary winding and said thyratron grid.

2. An alternating-current lighting system including two lamps to beseparately controlled, a thyratron connected in control relation to eachsaid lamp and to a power line, separate widerange phase-shift circuitsconnected to said power line and connected in control relation to saidthyratrons, respectively, whereby transients resulting from the earlyfiring of one thyratron appear in the power line, and a low-pass filterin the connection of each phase-shift network to said power line, and anadditional low-pass filter in the connection of each said thyratron tosaid phase-shift network, for preventing transmission of the transientsto the thyratron that is set for later firing.

3. An alternating-current lighting circuit having a lamp, a thyratronconnected in control relation to said lamp, said circuit being energizedby a power line, a grid-controlled vacuum tube and a condenser connectedas a resistancecapacitance circuit in control relation to the thyratrongrid and having energizing connections to said power line, anadjustable-amplitude input circuit for the grid of said vacuum tubeenergized directly by said power line, and separate low-pass filters inthe connection of the power line to said phase-shift circuit and in theconnection 'of 'said phase-shift circuit to said thyratron,respectively, whereby transients in the power line are prevented fromreaching the phase-shift network from the power line, and are preventedfrom reaching the thyratron input electrodes through the phase-shiftcircuit.

4. An alternating-current lighting system including separate lightingcircuits and lightingcontrol networks connected to a common power line,each lighting circuit including a thyratron connected in controlrelation to a lamp, whereby early firing of one thyratron producestransients in the power line to disturb the waveform available to thecontrol network of the other thyratron, and low-pass filters betweeneach control network and the power line and between each thyratron andits related control network, respectively.

5. An alternating-current lighting system including separatethyratron-controlled lighting circuits and lighting-control networkseach including a resistive voltage-divider and an adjustableresistance-capacitance phase-shift circuit, said lighting circuits andsaid phase-shift networks having energizing connections to a commonpower line, whereby early firing of a thyratron in one lighting circuitproduces transients in the power line, an input-filtering choke and anoutput-filtering resistancecapacitance network connected to eachphaseshift network for preventing transients in the power line producedby one lighting circuit from tripping the thyratron in the otherlighting circuit prematurely.

GEORGE C. IZENOUR.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,819,105 Machlett et a1. Aug.18, 1931 1,937,369 Willis Nov. 28, 1933 2,445,549 Wittenberg July 20,1948 Izenour Mar. 1, 1949

